A rotary slide valve is known from German Offenlegungsschrift No. 3,147,511. In this known rotary slide valve, in the normal position of the valve member the forward and return streams flow through an annular space between the outer cylinder and the inner fixed cylinder, and in the position in which the streams intersect, one stream flows through the first chamber of the valve member and the second stream flows through a second chamber thereof. The valve member rests by means of radial wall parts against the inner wall of the outer cylinder, that is to say the valve member interacts with the outer cylinder, so that the outer cylinder has to be produced with high precision. Since the four connection fittings are fastened to the outer cylinder, shear and tensile forces are easily transmitted from the connection pieces to the outer cylinder, with the result that there is no guarantee that the device will function reliably. Wear between the rotary valve member and the inner wall of the outer cylinder as well as jamming and leakages can result. Also, in offset position, the rotary valve member is subjected to uneven stress towards a tilted position because of the streams of water flowing through the two chambers, so that wear, jamming and leakage can occur and the bearings can be subjected to a severe load.